Throughout England people are now divided into Pures and Crazies according to the results of a DNA test, with the Pures living in small Communities cut off from the madness of society, and the Crazies living outside the walls in the squalor and mayhem of the City. Until the age of fifteen, Ana has lived a privileged existence amongst the Pures, but her whole world crumbles when she finds out that there was a mistake with her Pure test. She is actually one of the Crazies, and one day in the near or distant future she will become sick.

But Ana has already been promised to Pure-boy Jasper Taurell. Jasper is from a rich and influential family and despite Ana's defects, wants to be with her. The authorities grant Ana a conditional reprieve. If she is joined to Jasper before her 18th birthday she may stay in the Community until her illness manifests. But if Jasper changes his mind, she will be cast out among the Crazies. As Ana's joining ceremony looms closer, she dares to hope she will be saved from the horror of the City and live a 'normal' life. But then Jasper disappears.

Led to believe Jasper has been taken by a strange sect the authorities will not intefere with, Ana sneaks out of her well-guarded Community to find him herself. Her search takes her through the underbelly of society and into the pits of the human soul. And as she delves deeper into the mystery of Jasper's abduction she uncovers some devastating truths that destroy everything she has grown up to believe, but she also learns to love as she has never loved before.